This researh is concerned with the characterization of various proteoglycans synthesized by endothelial cells in culture and, in particular, with the isolation and characterization of heparan sulfate proteoglycans. For this purpose, an established line of endothelial cells derived from the intima of rabbit aorta has been used. 35S-proteoglycans are separated from other cell proteins by gel filtration; different proteglycan species are then fractionated by various electrophoretic techniques. Experiments in progress have shown that the spectrum of heparan sulfate proteoglycans synthesized by endothelial cell cultures is strikingly different from that of arterial smooth muscle cells with respect to both the chain size and the properties of the protein core.